Our investigations of the structure and function of the tryptophan synthase multienzyme complex provide insights into mechanisms of catalysis, metabolite channeling, and allosteric interactions. During the past year, we have investigated the effects of temperature, monovalent cations, and an allosteric ligand on the catalytic properties of tryptophan synthase. Our results provide evidence that increased temperature converts tryptophan synthase from a low activity, open conformation to a high activity, closed conformation under certain conditions. The allosteric ligand and monovalent cations affect the equilibrium between the open and closed forms. Mutations in a regulatory salt bridge between the alpha and beta subunits produce deleterious effects that can be repaired by increased temperature in the presence of certain ligands. To better understand the effects of disease causing mutations in human cystathionine beta-synthase, we have investigated the structure and function of cystathionine beta-synthase from yeast. Our results demonstrate that the N-terminal residues 1-353 compose a catalytic domain and that the C-terminal residues 354-507 compose a regulatory domain. Spectroscopic and kinetic studies establish the overall catalytic mechanism. Comparisons of the yeast and human enzymes reveal significant differences in catalytic and regulatory properties. Importantly, the human enzyme is heme-dependent, whereas the yeast enzyme is heme-independent.